By convention sweet is sweet, by convention bitter is bitter, by convention hot is hot, by convention cold is cold, by convention colour is colour. But in reality there are atoms and the void. That is, the objects of sense are supposed to be real and it is customary to regard them as such, but in truth they are not. Only the atoms and the void are real.

I am the most travelled of all my contemporaries; I have extended my field of enquiry wider than anybody else, I have seen more countries and climes, and have heard more speeches of learned men. No one has surpassed me in the composition of lines, according to demonstration, not even the Egyptian knotters of ropes, or geometers.

The first principles of the universe are atoms and empty space. Everything else is merely thought to exist. The worlds are unlimited. They come into being and perish. Nothing can come into being from that which is not nor pass away into that which is not. Further, the atoms are unlimited in size and number, and they are borne along in the whole universe in a vortex, and thereby generate all composite things-fire, water, air, earth. For even these are conglomerations of given atoms. And it is because of their solidarity that these atoms are impassive and unalterable. The sun and the moon have been composed of such smooth and spherical masses [i.e. atoms], and so also the soul, which is identical with reason.

Anaximenes and Anaxagoras and Democritus say that its [the earths] flatness is responsible for it staying still: for it does not cut the air beneath but covers it like a lid, which flat bodies evidently do: for they are hard to move even for the winds, on account of their resistance.

At first sight nothing seems more obvious than that everything has a beginning and an end, and that everything can be subdivided into smaller parts. Nevertheless, for entirely speculative reasons the philosophers of Antiquity, especially the Stoics, concluded this concept to be quite unnecessary. The prodigious development of physics has now reached the same conclusion as those philosophers, Empedocles and Democritus in particular, who lived around 500 B.C. and for whom even ancient man had a lively admiration.

Mock on, mock on, Voltaire, Rousseau!Mock on, mock on: 'Tis all in vain!You throw the sand against the wind,And the wind blows it back again.And every sand becomes a gemReflected in the beams divine;Blown back they blind the mocking eye,But still in Israel's paths they shine.The atoms of DemocritusAnd Newton's particles of lightAre sands upon the Red Sea shore,Where Israel's tents do shine so bright.

Democritus sometimes does away with what appears to the senses, and says that none of these appears according to truth but only according to opinion: the truth in real things is that there are atoms and void. 'By convention sweet', he says, 'by convention bitter, by convention hot, by convention cold, by convention colour: but in reality atoms and void.'

'By convention there is color, by convention sweetness, by convention bitterness, but in reality there are atoms and the void,' announced Democritus. The universe consists only of atoms and the void; all else is opinion and illusion. If the soul exists, it also consists of atoms.

In 'A Collection of Apophthegms, New and Old' (1625). As given in Essays, Moral, Economical, and Political: A New Edition, With the Latin Quotations Translated (1813), 'Contained in the original edition in octavo but omitted in later copies', No. 263, 353.

In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion.
(1987) -- Carl Sagan